Agricultural workers are often exposed to high concentrations of airborne microorganisms and at increased risk for developing respiratory diseases. Currently, little is known regarding the effectiveness of respirators used by agricultural workers especially with respect to airborne microorganisms. In the original grant, we developed a new method to measure workplace protection factors (WPF) of N95 filtering facepiece respirators against biological and non-biological particles. Our pilot-scale field study showed intriguing results on the effect of particle size and particle type on the WPF. Decreasing particle size decreased the WPF in a size range of 0.3 to 10 urn. Surprisingly, WPF-values against biological particles were 2-6 times lower than those measured for all dust particles in the same size range. The first aim of the proposed continuation is to generate a comprehensive field database on the WPF against biological particles and dust. Each subject (25 farmers or agricultural workers) will repeat WPF testing with two respirator models (one N-95 filtering facepiece and one elastomeric half-facepiece). The study will provide unique new data on the range of WPFs against particles of different characteristics in real field settings. The second aim is to investigate the effect of particle characteristics on their penetration into the respirator by manikin-based investigation and theoretical modeling. Particles from three selected farms will be characterized in regards to their aerodynamic size, shape, and microbiological composition. This testing is performed to select non-biological test particles and to isolate fungi and bacteria for further laboratory testing. Respirator performance will be compared for particles of different characteristics by manikin-based laboratory testing. Finally, manikin-based testing will be performed at three selected farms to assure realistic aerosol characteristics and composition. The combination of field and laboratory-based data and theoretical modeling will provide much-needed generalizable information to refine the guidelines on respirator use and to design more efficient respirators. This information can also be utilized for other work environments in which the respiratory protection is needed, such as moldy buildings, health care settings, and areas affected by bioterrorism. This proposal is well in line with NIOSH mission as the study outcome will help assure safe and healthful working conditions for agricultural workers through research, information, and training on respirators.